In a gravity-type of railroad classification yard, it is common practice to employ power operated braking apparatus, such as, hump and group frictional car retarders, to control the speed of the free-rolling cars as they move toward their given destinations in the various classification tracks. In practice, the railway car retarders employ a pair of braking elements, such as, brake beams and shoes or braking bars which are disposed parallel and on opposite sides of the track rail and which are movable between an opened nonbraking position and a closed braking position by a plurality of operating units. The operating units are substantially identical in construction and include a pair of pivotal levers which carry the braking elements. Each of the operating units includes a fluid pressure motor having a piston and cylinder for causing the pivotal levers to move the brake elements between the opened braking and closed nonbraking positions. Previously, the piston cylinder pressure motor was an extremely heavy assemblage which was very expensive to manufacture due to the excessive amount of machining that was required in making the cylinder member. In addition, the clyinder wall was susceptible to excessive wear and scoring which required the replacement of the cylinder member or the turning down of the interior surface of the cylinder wall. In either case, the frequent rebuilding of the fluid motors was not only costly in terms of machining or replacing the cylinders but also expensive from the standpoint of frequency of repair of the car retarder actuators. Further, it has been long recognized that it would be highly advantageous to effectively reduce the weight and overall cost of the fluid pressure motors. However, the high power handling requirements and the exigency to function properly under adverse environmental conditions have seriously deterred any significant changes or modifications in the basic design and construction of the conventional fluid pressure motors. Thus, it is essential that any proposed ramification must meet or exceed the long-standing criteria which has been established and expected of fluid motors for car retarders.